They found that standard 1980s chemotherapy treatments reduced breast cancer death rates by almost a quarter.

More effective modern drugs reduced mortality by about a third in a wide range of patients, compared with no chemotherapy.

The impact on death rates applied to all women irrespective of age, size of tumour, level of spread, and whether the cancer was fuelled by oestrogen.

In "ER-positive" cancers that are sensitive to oestrogen, chemotherapy plus hormone (endocrine) therapy was more effective than hormone treatment alone.

Sir Richard Peto, from Oxford University, one of the study leaders, said: "Most breast cancers are ER-positive, and for ER-positive disease that appears to have been completely removed by surgery, the 10-year risk of recurrence and death from breast cancer can be reduced by at least half by giving a few months of modern chemotherapy plus five years of endocrine therapy."

Chemotherapy can have severe side effects and so is only given if there is a substantial risk of cancer recurring.

The findings are published in an online edition of The Lancet medical journal.

Dr Susie Jennings, senior policy officer at the charity Breakthrough Breast Cancer, said: "Chemotherapy has transformed the outlook for breast cancer patients and this study provides clear proof of that. Even better, the benefits have increased over time due to newer, better drugs being used in smarter combinations."